This invention relates to a new process for the preparation of polyhydric alcohol 3-alkylthiopropionate ester stabilizers for polyolefin resins. Polyhydric alcohol 3-alkylthiopropionate ester stabilizers have been disclosed previously, but their preparation conventionally has required the use of relatively costly starting materials and furnished products that when used to stabilize polyolefin resins have contributed to the formation of undesirable yellowish and beige discolorations as well as unpleasant odors when the stabilized resins were processed or heat aged at elevated temperatures.
The pioneer disclosure of thioether carboxylic acid esters for stabilizing a polymer is believed to be by M. Gribbins in U.S. Pat. No. 2,519,755 of Aug. 22, 1950. Gribbins stabilized ethylene polymers with 0.001% to 5% by weight of a beta-thioether of an ester of propionic acid having the formula EQU ROOCCH.sub.2 CH.sub.2 S-X,
in which R is an alkyl or a cycloalkyl radical such as n- and isobutyl, amyl, heptyl, nonyl, decyl, lauryl, glycyl, cinnamyl, capryl, benzyl, allyl, cetyl, stearyl, palmityl, cyclohexyl, and similar groups, and X is:
1. a hydrocarbon group such as, e.g., the alkyl groups: methyl, ethyl, propyl, butyl, lauryl; the aryl groups: phenyl, naphthyl, benzyl; and such groups as p-methoxy phenyl, p-hydroxyphenyl and cyclohexyl.
2. an oxygenated-hydrocarbon group such as, e.g., the alcohol groups: hydroxymethylene, hydroxyethylene, and hydroxybutylene; the ether groups: methoxymethylene, methoxyethylene and ethoxyethylene; and acid groups and the R esters thereof: carboxymethylene, carboxyethylene, carboxypropylene and carboxybutylene; and aldehyde radicals as aldehydoethyl.
3. a sulfur-hydrocarbon group such as, e.g., mercaptoethyl, mercaptopropyl, mercaptobutyl, mercaptoisobutyl, mercaptohexyl and ethiaethyl.
4. a sulfur-and oxygenated-hydrocarbon group such as, e.g., carboxyethiaethyl (CH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 COOH).sub.2 carboxyethiaethyldithiaethyl (CH.sub.2 CH.sub.2 SSCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 COOH), carboxyethiaisobutyl (CH.sub.2 CH.sub.2 CH(CH.sub.3)SCH.sub.2 CH.sub.2 COOH) and carboxyethiapropyl (CH.sub.2 CH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 COOH).
5. a sulfur-and nitrogen-containing hydrocarbon group such as, e.g., 3-benzothiazyl mercaptopropionic acid, specifically described in U.S. Pat. No. 2,397,960.
Among these, Gribbins found the di-higher alkyl beta-thiodipropionates and especially the dilauryl and distearyl esters outstanding. Subsequently, thioether carboxylic acid esters and in particular thiodipropionates have been employed in conjunction with other stabilizers such as a polyhydric phenol in the stabilization of polypropylene and other polyolefins against degradation upon heating or ageing under atmospheric conditions. Disclosures by C. Tholstrup, U.S. Pat. Nos. 3,033,814 of May 8, 1962 and 3,160,680 of Dec. 8, 1964; L. Rayner, U.S. Pat. No. 3,181,971 of May 4, 1965; D. Bown, U.S. Pat. No. 3,242,135 of Mar. 22, 1966; S. Murdock, U.S. Pat. No. 3,245,949 of Apr. 12, 1966; H. Hagemeyer, U.S. Pat. No. 3,282,890 of Nov. 1, 1966; J. Casey, U.S. Pat. Nos. 3,496,128 of Feb. 17, 1970 and 3,586,657 of June 22, 1971; M. Minagawa, U.S. Pat. Nos. 3,549,572 of Dec. 22, 1970, 3,629,189 of Dec. 21, 1971, 3,673,152 of June 27, 1972, 3,849,370 of Nov. 19, 1974 and 3,869,423 of Mar. 4, 1975; W. Drake U.S. Pat. No. 3,624,026 of Nov. 30, 1971; A. DiBattista, U.S. Pat. No. 3,824,192 of July 16, 1974; B. Cook, U.S. Pat. No. 3,850,877 and H. Mueller U.S. Pat. No. 3,850,918 of Nov. 26, 1974; M. Dexter U.S. Pat. Nos. 3,856,748 of Dec. 24, 1974, 3,888,824 of June 10, 1975, and 3,903,160 of Sept. 2, 1975; P. Klemchuk U.S. Pat. No. 3,860,558 of Jan. 14, 1975; M. Rasberger U.S. Pat. Nos. 3,867,340 of Feb. 18, and 3,901,931 of Aug. 26, 1975; H. Brunetti U.S. Pat. Nos. 3,867,337 of Feb. 18 and 3,873,498 of Mar. 25, 1975; S. Rosenberger U.S. Pat. Nos. 3,884,874 of May 20 and 3,887,518 of June 3, 1975; C. Ramey U.S. Pat. No. 3,907,803 of Sept. 23, 1975 are representative of a very large number of stabilizer combinations including dilauryl and distearyl thiodipropionate or other dialkyl thiodipropionates along with polyhydricphenols and sometimes organic phosphites, metallic stearates, ultraviolet absorbers, nickel compounds, and heavy metal deactivators for use in polypropylene and other polyolefins.
While dialkylthiodipropionates have many favorable attributes such as availability in high purity at reasonable cost, a low degree of toxicity, and generally good stabilizing effectiveness, certain problems attendant on their use have long been recognized, particularly the need to use high concentrations in certain highly stressed formulations to obtain the required heat stability, and a tendency to lose effectiveness in use as a result of exposure to the leaching action of moving streams of warm water and warm air as in the washing and drying cycles of automatic dishwashers and laundry machines.
Attempts to improve on these characteristics have included the use of more efficient and more permanent thiodipropionate esters as well as more effective antioxidants and stabilizer combinations. Thus A. Hecker in U.S. Pat. No. 3,244,650 of Apr. 5, 1966 disclosed a stabilizer system for polypropylene composed of three stabilizers; an organic polyhydric phenol, an organic phosphite and a polyvalent metal salt of an organic acid. To this system, U.S. Pat. No. 3,255,136 of June 7, 1966 added a fourth ingredient, a thiodipropionic acid ester having the formula: EQU R.sub.1 OOCCH.sub.2 CH.sub.2 --S--CH.sub.2 CH.sub.2 COOY
in which R.sub.1 is an organic radical selected from the group consisting of hydrocarbon radicals such as alkyl, alkenyl, aryl, cycloalkyl and mixed alkyl aryl and mixed alkyl cycloalkyl radicals; hydroxyalkyl and hydroxyalkyloxyalkylene radicals; and esters thereof with aliphatic carboxylic acids; and Y is selected from the group consisting of (a) hydrogen, (b) a second R radical R.sub.2, which can be the same as or different from the R.sub.1 radical, (c) a polymeric chain of n thiodipropionic acid ester units: EQU --XO(OCCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 COOXO).sub.n OCCH.sub.2 CH.sub.2 --S--CH.sub.2 CH.sub.2 COOZ
where Z is hydrogen, R.sub.2 or M, n is the number of thiodipropionic acid ester units in the chain, and X is a bivalent hydrocarbon group of the type of R.sub.1, that is, alkylene, alkenylene, cycloalkylene, mixed alkylene-arylene and mixed alkylene-cycloalkylene radicals: hydroxyalkylene and hydroxyalkyloxyalkylene radicals; and esters thereof with aliphatic carboxylic acids; and (d) a polyvalent metal M of Group II of the periodic table such as zinc, calcium, cadmium, barium, magnesium and strontium.
The molecular weights of the R and Y radicals are taken such that with the remainder of the molecule the thiodipropionic ester has a total of from about ten to about sixty atoms per sulfur atom.
U.S. Pat. No. 3,378,516, patented Apr. 16, 1968 to Tholstrup, Bell and Kibler, proposes combinations including linear thiodi alkanoate polyesters obtained from a thiodialkanoic acid and a diol having a molecular weight of from about 500 to 4000, together with a phenolic antioxidant and/or a phosphite. These combinations are said to display synergistic stabilizing effectiveness.
H. Schirmer in U.S. Pat. No. 3,598,776 of Aug. 10, 1971, disclosed that the incorporation of 10% by weight disproportionated resin in polypropylene enabled him to use 2% by weight dilaurylthiodipropionate (LTP) in the polymer without blooming while in the absence of the rosin only 1% could be used without blooming and the stability of the polymer with the rosin at the higher LTP concentration was significantly increased.
H. Schutze in U.S. Pat. No. 3,630,991 of Dec. 28, 1971 disclosed non-exuding and non-volatile sulfur containing esters of cyclic terpene alcohols for the stabilization of 2 to 8 carbon alpha-olefin polymers together with hindered phenols. Schutze's esters may be represented by the structural formulae ##STR1## where R.sub.a is --CH.sub.2 (CH.sub.2).sub.n COOR' or alkyl EQU n=1 to 5 EQU m=1 to 16
R is a radical selected from the group consisting of abietyl, hydroabietyl, tetrahydroabietyl, dihydroabietyl, dehydroabietyl, dihydropimaryl, tetrahydropimaryl, borneyl, alpha-terpineyl, B-terpineyl, V-terpineyl, methyl, and dihydroterpineyl, and R' is a radical selected from the group consisting of abietyl, hydroabietyl, tetrahydroabietyl, dihydroabietyl, dehydroabietyl, dihydropimaryl, tetrahydropimaryl, borneyl, alpha-terpineyl, B-terpineyl, methyl, and dihydroterpineyl.
A. Onishi, in U.S. Pat. No. 3,629,194 of Dec. 21, 1971 disclosed a polyolefin resin stabilized against thermal aging with esters of alkyl thiopropionic or alkyl thiobutyric acid with a polyol having up to five hydroxyl groups, in combination (optionally) with a phenolic antioxidant. The alkyl thiopropionic or alkyl thiobutyric acid esters are defined as having one of the formulae: EQU (1) R--SC.sub.n H.sub.2n COOR'OOCC.sub.n H.sub.2n SR EQU (2) RSC.sub.n H.sub.2n COOC.sub.m H.sub.2m SC.sub.m H.sub.2m OOCC.sub.n H.sub.2n SR EQU (3) R"C--CH.sub.2 OX).sub.3 ##STR2## and EQU (5) --C--CH.sub.2 OX).sub.4
wherein
R is an alkyl of 8 to 30 carbon atoms, PA1 m and n are each integers of 2 or 3, PA1 R' is an alkylene containing 2 to 12 carbon atoms, PA1 R" is an alkyl containing 1 to 20 carbon atoms, PA1 X is hydrogen or --OC--C.sub.n H.sub.2n SR, at least one of which is --OCC.sub.n H.sub.2n SR,
the R.sub.1,R' and R" moieties in one compound being the same or different.
The phenolic antioxidants are defined by Onishi as mono- or polyhydric phenolic compounds in which at least one of the ortho positions to a hydroxyl group is substituted by an alkyl, aralkyl, or cycloalkyl group.
The substituents preferably contain carbon atoms of a number of the order of 3 to 10, and the alkyl group, inclusive of that in an aralkyl and cycloalkyl groups can be unsaturated. The phenolic compounds may be further substituted, and the phenolic compounds may be polyphenolic compounds such as bisphenolic, trisphenolic, or tetrakisphenolic compounds in which phenolic nuclei are connected by a connecting group such as an alkylene, a thioether, or a triazinoxyl group.
M. Dexter in U.S. Pat. No. 3,758,549 of Sept. 11, 1973 disclosed alkyl esters derived from alkyl thioalkanoic acids and alkane polyols, such as pentaerythritol tetrakis, 3-n-dodecylthiopropionate, and ethylene-bis-3-n-dodecylthiopropionate. These are used in combination with phenolic antioxidants to effectively stabilize polyolefins from the deleterious effects of heat and oxygen. The alkyl esters are defined by the formula: ##STR3## wherein
R is an alkyl group of from one to eighteen carbon atoms,
n has a value of from 2 to 4; and
Z is an aliphatic hydrocarbon of the formula: EQU C.sub.y H.sub.2y +2-n
in which y has a value of from 2 to 18 when n is 2 and a value of from 3 to 6 when n is greater than 2, the value of y in all cases being equal to or greater than that of n.
M. Minagawa in Japanese Kokai No. 75/106881 of Aug. 27, 1975 disclosed stabilized resin compositions containing 3-alkylthiopropionate esters of alcohols containing a nitrogen-heterocyclic ring, for example tris(2-hydroxyethyl isocyanurate) and optionally a phenolic antioxidant.
E. Schurdak in U.S. Pat. No. 3,966,675 of June 29, 1976 has disclosed mixtures of pentaerythritol tetrakis (3-n-dodecylthiopropionate) with bis(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl) dithiolterepthalate or 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)-s-triazine-2,4,6-trione that are stated to be extremely effective in inhibiting the thermal degradation of polypropylene.